I've paired the Mojo with the likes of the Oppo HA-1, Liquid Carbon, and Liquid Gold. With these amps I've only used a very small portion of the volume to drive my headphones. In some regards they sound great - added soundstage, more bass oomph, sparkling treble, etc. The biggest difference is that when my headphones are driven direct from the Mojo (and the DAVE) there is more variety to the music. They don't add the same colouration to everything. Not all songs are wider in the soundstage, not all the bass is phat, not all the treble is emphasized. There is more variance from hard sounds to soft sounds. More accurate timbre of the individual notes IMO. That's what I love about the Mojo, the fact that it is very transparent and lets the music shine, not just the flavour of the amp. Nothing wrong with either option (that's why I own the Liquid Gold), but it's really a preference and not anything to do with capability to drive most headphones. That's my 2 cents. YMMV.
Yes, it's impossible to add a component and also add transparency. 1+1 =/= 0.
I think it's funny how the hd650 is always the benchmark for the Mojo when the Mojo is criticized. Good grief, it's not the only headphone out there and synergy, again, is simply not the same as drive ability. Perhaps the hd650 isn't the end all be all headphone that's out there. I'm glad you like the hd650 from your amp, in the end that's what matters, the smile on your face as you liZ